A longstanding goal of cancer research has been to stimulate the immunological rejection of tumors. This goal is based on the hypothesis that tumors express foreign antigens which can potentially serve as targets for the immune system (Himmelweit (1957) The Collected Papers of Paul Ehrlich, Pergamon Press, Oxford, England). Although it remains controversial to what extent spontaneous tumors express antigens which can be recognized as foreign by the immune system in conventional immunization and challenge experiments (Hewitt et al. (1976) Br. J. Cancer 33:241-259), it is well documented that many experimental tumors express antigens which can mediate tumor rejection in such experiments (Hellstrom and Hellstrom (1991) Principles of Tumor Immunity: Tumor Antigens, In: The Biologic Therapy of Cancer. DeVita, Jr. et al., Eds., J. B. Lippincott Co., Philadelphia, pp. 35-52; Boon (1992) Adv. Cancer Res. 58:177-211).
Cellular immunity, primarily mediated by T lymphocytes, plays the key role in the rejection of antigenic tumors. Both T helper cells (Th) and cytolytic T lymphocytes (CTL) are involved (Melief (1992) Adv. Cancer Res. 58:143-175; Greenberg (1991) Adv. Immunol. 49:281-355). Recognition and destruction of immunological targets require T lymphocyte recognition via the T cell receptor (TCR) of antigenic peptides presented in the context of MHC molecules (Bjorkman et al. (1988) Nature 329:512-518; Unanue (1984) Annu. Rev. Immunol. 2:395; Townsend et al. (1986) Cell 44:959-968). Although T cell immunity has been detected against specific tumor antigens in some animals and humans (van der Bruggen et al. (1991) Science 254:1643-1647; van den Eynde et al. (1991) J. Exp. Med. 173:1373-1384; Anichini et al., (1987) Immunol. Today 8:385-389), these "imunogenic" tumors generally grow progressively and eventually kill their hosts.
There are several reasons why even those tumors which express rejection antigens can evade immune destruction. They include the failure of tumors to adequately process and present antigens to T cells because of reduced levels of MHC class I expression (Elliot et al. (1989) Adv. Cancer Res. 53:181-244). A problem which might be circumvented by transfection with MHC class I genes (Hui et al. (1984) Nature 311:750-752; Tanaka et al. (1984) Science 228:26-30; Wallich et al. (1985) Nature 315:301-305) or with .gamma.-interferon DNA which enhances antigen processing (Restifo et al. (1992) J. Exp. Med. 175:1423-1431). Lack of an effective antitumor immune response may also result from a deficiency in tumor-bearing animals of T helper functions necessary both for the clonal expansion of tumor-specific CTL (Fearon et al. (1990) Cell 60:397-403) and for the activation of macrophages and other inflammatory cells that can cause tumor destruction. Transfection of tumor cells with IL-2 or IL-4 cDNAs result in paracrine IL-2 secretion of lymphokines which substituted for T cell help, induced tumor-specific CTL, and cause tumor rejection (Fearon et al., (1990) Cell 60:397-403; Gansbacher et al. (1990) J. Exp. Med. 172:1217-1224; Ley et al. (1991) Eur. J. Immunol. 21:851-854; Golumbek et al. (1991) Science 254:713-716). Similarly, transfection of tumors with IL-4 cDNA can also cause tumor rejection (Tepper et al. (1989) Cell 57:503-512) and the generation of T cell-mediated tumor immunity (Golumbek et al. (1991) Science 254:713-716).
Another mechanism which may contribute to the induction of efficient tumor-reactive T cells is implicated from the two-signal models for immune cell activation. This model was originally proposed for B lymphocytes (Bretscher and Cohn (1970) Science 169:1042-1049) as an explanation for why antigens expressed on cells of nonhematopoetic origin are ineffective in inducing transplant rejection (Lafferty et al. (1983) Ann. Rev. Immunol. 1:143). Two-signal models have now been extended to all lymphocytes (Janeway (1989) Cold Spring Harbor Symp. Quant. Biol. 54:1-13; Nossal (1989) Science 245:147-153; Schwartz (1989) Cell 57:1073-1081). These models postulate that lymphocytes require for optimal activation both an antigen specific signal delivered through the antigen receptor, and a second, antigen non-specific or costimulatory signal. T cell costimulatory pathways determine whether TCR complex engagement results in immune cell activation or inactivation (Mueller et al. (1989) Annu. Rev. Immunol 7:445; Schwartz (1989) Cell 57:1073-1081) and antigen presentation in the absence of T cell costimulation leads to functional inactivation or clonal anergy or even cell death (Schwartz, (1989) Cell 57:1073-1081).
The molecular basis of T cell costimulation is not well understood, but may involve several molecules on antigen presenting cells (APC) which are recognized by T cell surface receptors (van Seventer et al. (1991) Curr. Opinion Immunol. 3:294-303). One important costimulatory molecule is B7 which is expressed on activated B cells (Freeman et al. (1989) J. Immunol. 143:2714) and other APC (Freeman et al., 1989; Razi-Wolf et al. (1992) Proc. Natl. Acad. Sci. USA 89:4210-4214). B7 binds to the CD28 (Linsley et al. (1990) Proc. Natl. Acad. Sci. USA 87:5031-5035) and CTLA-4 (Linsley et al. (1991) J. Exp. Med. 173:721-730) receptors on T cells and costimulates proliferation of human and murine CD4.sup.+ T cells (Linsley et al. (1991) J. Exp. Med. 174:561-569; Gimmi et al. (1991) Proc. Natl. Acad. Sci. USA 88:6575-6579; Kuolova et al. (1991) J. Exp. Med. 173:759-762; Damle et al. (1992) J. Immunol. 132:1985-1992). Experiments in vitro suggest that signals transduced by the CD28 receptor (June et al. (1990) Immunol. Today 11:211-216) can determine whether TCR occupancy results in a productive immune response or clonal anergy (Jenkins et al. (1991) J. Immunol. 147:2641-2466; Harding et al. (1992) Nature 356:607-609). Experiments in vivo indicate that blocking costimulation by B7 can effectively suppress humoral responses (Linsley et al. (1992) Science 257:792-795) and make possible long-term acceptance of tissue xenografts (Lenschow et al. (1992) Science 257:792-795).
The B7 molecule is expressed primarily on hematopoetic cells (Freeman et al. (1989) J. Immunol. 143:2714) and it is present only at very low levels, if at all, on many cultured human tumor cell lines. These findings suggest that one of the reasons why immunogenic tumors can often escape T cell destruction is that they lack appropriate costimulatory molecules. A prediction from this hypothesis is that introduction of costimulatory molecules into tumors which possess tumor rejection antigens would enhance their ability to induce specific anti-tumor immunity leading to tumor eradication in immunocompetent hosts. This hypothesis was tested using a murine model for tumors which express "rejection" antigens but which nonetheless grow progressively in their hosts. In this model, the human papillomavirus 16 (HPV-16) E7 gene was transfected into the poorly immunogenic K1735-M2 melanoma (Fidler and Hart (1981) Cancer Res. 41:3266-3267). A tumorigenic transfectant, E7C3, was selected, against which a CD8.sup.+ cell-mediated and HPV-16 E7-specific tumor immunity can be generated, by immunization of synegeneic C3H/HeN mice with E7-expressing fibroblasts (Chen et al. (1991) Proc. Natl. Acad. Sci. USA 88:110-114; Chen et al. (1992) J. Immunol. 148:2617-2621). The present invention demonstrates that transfection of E7C3 tumor cells with the costimulatory molecule B7 induces antitumor immunity to E7.sup.+ tumors.